A molding system, such as an injection molding system, forms molded articles from a molding material. The molding material may be a plastic or resin material, such as Polyethylene Terephthalate (PET) for example. The molded article may be a container, or a container precursor such as a preform capable of being subsequently blown into a beverage container (e.g. a plastic bottle).
An injection molding system may heat a molding material, such as PET, to a homogeneous molten state, in which state the molding material may be referred to as “melt.” The melt may be injected, under pressure, into a molding cavity that is defined by a collection of components referred to as a “mold stack.” The mold stack typically includes, among other components, a female cavity piece and a male core piece attached to a cavity plate and a core plate respectively. The molding cavity that is defined by the mold stack may have a shape that substantially corresponds to a final cold-state shape of the article to be molded.
During injection of melt, a clamp force is applied to the cavity and core plates that is sufficient to keep the cavity and the core pieces together despite the opposing force of the pressurized melt within the molding cavity. Once the molding cavity has been filled with melt, the molded article is typically allowed to cool and harden within the molding cavity for a brief period of time. Cooling may cause the molded article to shrink within the molding cavity such that, when the cavity and core plates are urged apart, the molded article may remain associated with the core piece. The molding system may use various types of ejection structures to assist in removing the molded article from the core piece. Examples of ejection structures include stripper plates and ejector pins.
A molded article such as a preform may have a neck portion (or “neck finish”) having various features in relief. The neck portion features may include one or more of: threads for accepting and retaining a closure assembly (e.g. a bottle cap); an anti-pilferage assembly configured to cooperate with the closure assembly to indicate whether the end product (e.g. a beverage container filled with a beverage) has been tampered with; and a support ledge that cooperates with parts of the molding system. The relief of these features is such that removal of the neck portion from a molding cavity defined by a unitary female cavity piece would be difficult or impossible. For this reason, the neck portion is typically defined by a split mold insert (also referred to as a neck ring) designed to separate laterally into two or more parts/halves to release the neck portion of the cooled molded article for axial ejection from the core piece.
At the beginning of an injection molding cycle, a molding cavity is empty, i.e. filled with air. As melt is injected, the melt progressively replaces the air within the molding cavity. The air is typically vented from the molding cavity through vents that are defined between mold stack components at or near the end of the melt flow path within the molding cavity. Vents may be sized to permit passage of a gas (normally air) therethrough without permitting passage of melt therethrough. The vent sizes may be set based on the type and/or viscosity of the melt to be used. For example, in the case where the molding material is PET, the vents may comprise gaps approximately 30 to 40 microns wide. The venting may promote molded article quality by reducing or eliminating a risk of trapped air within the molding cavity, which might otherwise cause imperfections in the molded article.
When an injection molding system is operated over many molding cycles, a residue may accumulate on vent surfaces. The residue may for example be made up of material dust, contaminants or other particles. An excessive buildup of such residue may prevent air from being properly or completely vented from the molding cavity, which may jeopardize the quality of the molded article.